This invention concerns a method for preparing aqueous kaolin clay slurries of reduced low shear viscosity.
Kaolin clay has found many uses in the chemical and related industries. For example, it is employed extensively by the paper industry as a filler and coating pigment. For such uses, however, the naturally occurring kaolin must be refined.
In the typical refining procedure, the crude kaoline is slurried in water to a solids content of from about 35 to 45 weight percent in the presence of a primary dispersant such as sodium hexametaphosphate. The resulting slurry is screened to remove grit and then subjected to delamination and centrifugal classification to yield a fine fraction containing primarily particles of less than two microns. The fine fraction is flocculated with such as alum and sulfuric acid, bleached with such as sodium hydrosulfite, filtered and washed with water. The filtered refined clay, which has a solids content of about 60 weight percent, is then either dried to a powdered product, such as by spray drying, or redispersed in water in the presence of a secondary dispersant to a slurry having a solids content of from about 60 to 72 weight percent, using dried product as needed.
Numerous secondary dispersants have been disclosed for the dispersion of refined clay. The condensed phosphates such as sodium tripolyphosphate and tetrasodium pyrophosphate have been found effective and are commonly used for this purpose. However, these agents tend to readily degrade by hydrolysis in water, causing the clay slurry to thicken and finally gel on standing. Dispersants purported to alleviate this problem include citric acid as disclosed in U.S. Pat. No. 4,144,083 and sodium polyacrylate as disclosed in Canadian Pat. No. 854,271. Sodium polyacrylate has further been disclosed in U.S. Pat. No. 3,736,165 for use as a primary dispersant which is purported to carry through the clay refining process and act along with the added secondary dispersant such as citric acid to disperse the refined clay.
It is the primary objective of the present invention to provide a more effective dispersant system for preparing a refined kaolin clay slurry of reduced viscosity and improved viscosity stability.